The present invention is related generally to microwave systems, and, more particularly, to transparent high-power windows used in the millimeter region.
Microwave systems often require windows that are transparent at the frequencies of interest. This problem is particularly acute at millimeter-wave frequencies, where most dielectric materials tend to have high loss tangents. At low power levels, a high loss tangent may be acceptable, as long as the window is thin enough to prevent more than a small fraction of the incident power from being absorbed. At high power levels, a window made from a material having a high loss tangent will become extremely hot and may fail if not actively cooled. Such windows are usually cooled at their edges, since most coolants themselves have high loss tangents and therefore cannot be directly exposed to millimeter-wave power. The need therefore exists for a microwave window capable of reliably transmitting extremely high levels of millimeter-wave power.
Surface-cooled double-disk windows made from sapphire have been used as the output windows for high-power gyrotrons. These windows are cooled by a special coolant having a low loss tangent at millimeter-wave frequencies. The coolant flows in the gap between the two disks. While double-disk windows improve upon the performance of single-disk edge-cooled windows, their thermal performance is insufficient to allow megawatt-class gyrotrons designed for CW operation to operate for more than a few seconds at a time.
Recently, synthetic diamond disks of sufficient size and quality for use as gyrotron output windows have become available. Diamond is a nearly ideal material for use as a dielectric window, as the loss tangent of high-quality material is very low at millimeter-wave frequencies ( less than 5xc3x9710xe2x88x925) and its thermal conductivity is twice that of copper. However, because a disk of sufficient size and thickness for a gyrotron window takes several weeks to grow, and because there are few sources for such disks, diamond windows are very expensive.
Thus, there remains a need for transparent windows at millimeter frequencies that avoid most, if not all, of the problems described above.
In accordance with the present invention, a millimeter-wave window is constructed from a high conductivity metal such as copper, beryllium copper, or aluminum. The metallic plate is made transparent over a range of frequencies by perforating it with a periodic array of slots, or openings.
In one embodiment, the millimeter-wave window of the present invention is used as the output window in a gyrotron. In such a case, one suitable periodic array of slots, or holes, comprises an equilateral triangular array of slots. By proper choice of the hole spacing and diameter, the window can be made transparent at any desired frequency.
In addition to being transparent, however, the output window must also be vacuum tight, as the pressure inside a gyrotron must be maintained at a level on the order of 10xe2x88x929 torr. The present invention solves this problem by covering the surface of the high-pressure side of the window with a thin layer of a suitable dielectric material. A suitable dielectric will have a low loss tangent and a low coefficient of thermal expansion. In addition, if the dielectric is to be used in a high-vacuum environment, it must be of a material that does not continuously evolve gasses from its surface (ruling out the use of most polymers and organic-based materials). Materials suitable for use in a high-vacuum environment include alumina, fused quartz, sapphire, and CVD diamond. For applications in which the window must provide an air-tight seal but is not required to maintain a high vacuum, the last requirement on the window material can be relaxed. Because the dielectric is in intimate contact with the perforated metal plate, any heat generated in the dielectric layer has only to diffuse to the dielectric-metal boundary, where it is quickly carried away by conduction in the much higher conductivity metal. As a result, the dielectric need not have a high thermal conductivity. For most applications, edge cooling of the metal-dielectric window should provide sufficient cooling. For very high-power applications where edge cooling may be inadequate, cooling channels may be incorporated directly into the interior of the perforated metal plate, which will allow the window to transmit more power than its edge-cooled counterpart.
The novel features of the present invention are its use of a periodic metal structure as a high-power microwave window. Metal structures have been used in windows before, but usually in such a way so as not to interfere with the transmission of microwave energy; this is typically done by placing thin metal ribs perpendicular to the incident electric field. The present invention takes a different approach by making a metal structure an integral part of the window, one that strongly interacts with the incident microwave fields. This approach toward window design is considered to be novel and unique.